The invention relates generally to superconducting magnet systems and more particularly to low AC loss thermal shields of a superconducting magnet system.
In one example, an MR system includes a cold mass that comprises a superconducting magnet, a magnet coil support structure, and a helium vessel. Liquid helium contained in the helium vessel provides cooling for the superconducting magnet and maintains the superconducting magnet at a low temperature for superconducting operations, as will be understood by those skilled in the art. The liquid helium maintains the superconducting magnet approximately and/or substantially at the liquid helium temperature of 4.2 Kelvin (K). For thermal isolation, the helium vessel that contains the liquid helium in one example comprises one or more thermal shields and a vacuum vessel.
The vacuum vessel maintains a vacuum environment that eliminates convection heat loads. The thermal shield intercepts radiation and conduction heat loads to the cold mass. Conventional thermal shields are made of conductive metals such as copper or aluminum. The thermal shield is cooled to an intermediate temperature between 4.2 K and the room temperature by a cryocooler or some cryogen such as the liquid nitrogen. It completely surrounds the 4.2 K cold mass to block radiation heat from the room temperature vacuum vessel to the cold mass. Good thermal conduction is required for the thermal shield to make its temperature as low as possible. When the magnet is operated in an AC field, such as during MR imaging, eddy currents will be induced in the thermal shield components. The eddy current generates heat in the thermal shield that must be removed by the cryogenic system. Also, during a magnet quench, as the current of the magnet coil rapidly decays to zero, large eddy currents and quench forces will be induced in the thermal shield. It is difficult for the thermal shield to withstand the quench forces. It is also difficult to structurally support the thermal shield under the quench forces.
It would therefore be desirable to have an apparatus configured to reduce quench forces and eddy currents generated by AC fields in the thermal shield.